From William Harvey to Giraffes to Aerospace: The Cerebral Circulation is a Circle, Not a Line, and its Normal Position is Upright. Is There a Conceptual or Physical Model That Accommodates Those Realities?

November 13th, 2018JAMES MUNIS
Mayo Clinic School of Medicine, Rochester

In 1628, William Harvey taught us that the systemic circulation is a circle, not a line. In our own time, Harvey’s observation has been challenged by the promotion of an alternative model of the cerebral circulation that contains “open,” rather than “closed” hydraulics. That is, the challenging model assumes a circulatory path based on a conceptual model of either an open-air waterfall or a so-called “Starling Resistor.” Nonetheless, the actual anatomy of the cerebral circulation is a continuously patent circuit that accommodates 750 mL/min blood flow, regardless of posture. What kind of conceptual or physical model is the most realistic, both anatomically and physiologically? The observation that the cerebral circulation is continuously patent provides evidence against a Starling resistor model. The observation that it is a closed path and does not empty into the open-air is also evidence against a waterfall model. An adequate model of the cerebral circulation should also accommodate the observations that it functions in any posture (including upside down), that it functions in weightlessness during space flight, that it can explain the phenomenon of sub-atmospheric pressure and venous air embolism, and that the normal posture of humans is upright. This lecture will outline a closed model of the cerebral circulation that satisfies those anatomic and physiologic realities, and that may provide a more realistic and robust conceptual framework for empiric measurements of cerebral blood blow in any posture or gravitational state.